The present invention is directed to a self-tapping screw for use in hard receiving materials, such as concrete, masonry, gas concrete and the like with cutting threads extending around the screw shank. An annular enclosing space is defined around the screw shank by the thread tip with the space having a cylindrical shape along an axially extending part of the screw shank and then tapering inwardly toward the screw tip so that the axial ends of the inwardly tapering enclosing space is located within an axially extending section of the tapering leading end of the screw with the leading end of the cutting thread forming the leading end of the enclosing space.
Screws of the above type are used in a self-tapping or self-cutting operation in different receiving materials. Such receiving materials can be concrete, masonry, gas concrete and the like, covering a spectrum ranging from porous to harder materials. With harder material, such as concrete, the setting operation occurs initially by preboring to a diameter corresponding approximately to the diameter of the core or outer surface of the screw shank. In porous material, the screws are set or installed directly, that is, without preboring. If a screw is set without preboring, there is a considerable time saving in the setting operation. As a result, the screws under discussion have been successful in situations where they can be set without preboring, that is, in the various types of gas concrete presently in use.
Since there is an increasing use of gas concrete, especially in the rehabilitation of older buildings, the demand for simple and reliable settable screws, which can be installed in a self-tapping manner, is on the increase. Such screws are used for fastening all kinds of objects, such as wall cabinets, pipe hangers, ceiling hangers, appliances and many others.
A main requirement for self-tapping screws is that they can be threaded into a receiving material without causing resistance to any great extent. This requirement is not satisfied by most of the screws presently in use, since the core of the screw and the tapping threads are in such a relationship to one another, that there is a considerable increase in resistance after the first revolution during the setting of the screw. Installing such screws without exerting the required effort or force is thus impossible. The effort required by operating personnel results in a cumbersome setting procedure and, in addition, in considerable irregularities in the penetration of the screw into the receiving material, whereby the fastening quality suffers greatly because of this last-named disadvantage. Moreover, the fastening quality is especially questionable, if interruptions in the application of the setting force occurs due to fatigue by the operating personnel.
It has been found that the disadvantage of such increasing resistance during setting screws can be avoided, if the leading end of the screw is configured as a so-called "corkscrew tip". The characteristics of such a screw end results in the core tapering toward the screw tip at an angle which is considerably smaller than the angle of the enclosing space formed by the thread tips of the tapering part of the cutting thread. A self-tapping screw with a leading end formed in this manner is disclosed in U.S. Pat. No. 4,536,117.
The screw disclosed in this patent can solve the disadvantages involved in the increasing resistance when the screw is threaded in, however, it has another significant disadvantage. This disadvantage is that the leading end of the cutting thread extending from the screw core occurs asymmetrically and, as a result, the leading end of the screw is is urged to one side when penetrating into the receiving material causinh an irregular hunting motion. This hunting motion can continue during further threading-in of the screw, whereby the cutting thread insufficiently penetrates into the receiving material, resulting in an inadequate fastening quality. Moreover, the irregular hunting motion is transmitted to the driving device, such as a hand-held drilling tool, so that in addition to the vibration phenomenon which is inconvenient for operating personnel, more or less permanent damage can occur in the drilling device.